The present invention relates to a method and apparatus for backing up a master station of a data transmission system having the master station and a slave station, and more particularly to a method and apparatus for preventing an overrun when a failure occurs in the master station or service unit in a POS (point of sales) system or computer network systems.
In a data transmission system, it is common practice to connect a service unit (master station) and a backup unit to a data transmission line in common to terminals (slave station) and use the backup unit when the service unit is down, in order to enhance reliability. In a prior art system of this type such as a daisy chain type disclosed in JP-A-No. 57-81655 laid open on May 21, 1982, a signal line for controlling switching between the service unit and the backup units, in addition to the data transmission line, is connected between the service unit and the backup units, and an inhibit signal is supplied to the signal line to switch the service unit and the backup unit. In this system, each backup unit requires the signal line and a logic unit to control it. Namely, the prior art system needs a number of signal lines and logic units having complex logic functions such as for distinguishing between down state of the service unit and power-off state. This effects in cost increase. Further, because of a noise on the signal line, the logic unit may cause malfunction. Because it is not possible to prevent an overrun signal, due to a failure of the service unit, from being transmitted to the data transmission line, the reliability to the backup operation is not sufficient and the influence- of overrun to the slave station cannot be adequately prevented.